Fuel control for a dual turbine power plant



Oct. 27, 1959 T. P. FARKAS 2,909,895

FUEL CONTROL FOR A DUAL TURBINE POWR PLANT 2 Sheets-Sheet l Filed Aug. 17, 1955 Oct. 27, 1959 T. P. FARKAS 2,909,895

FUEL CONTROL FOR A DUAL TURBINE POWER PLANT Filed Aug. 1'7, 1955 2 Sheets-Sheet 2 United States Patent I FUEL CONTROL FOR A'DUAL TURBlNE 5 POWER PLANT Thomas P. Farkas, Bloomfield, Conn., assignor to United Aircraft Corporation, East Hartford, Conn., a corporation of Delaware Application August 17, 1955., Serial No. 528,879 9 Claims. (Cl. 60-39.16)

This invention relates to fuel controls and more particularly to fuel controls for turbine type power plants.

It is an object of this invention to provide a fuel control for a turbine power plant having multiple, disconnected turbine stages. 2O

lt is a further object of this invention to provide a fuel control of the type described whereby fuel ow is regulated under different operating conditions in proportion to a plurality of parameters or variables of power plant operation including the speed of each of said turbine stages.

It is another object of this invention to provide a fuel control which can regulate fuel flow for the particular arrangement of a power plant having one turbine driving a compressor and a free turbine for driving a propeller or helicopter rotor or the like.

Another object of this invention resides in the provision of a fuel control which utilizes the speed of one of the turbine stages for steady state and overspeed operation lwhile the speed of the other turbine stage is utilized for ticularly adapted to controlling a turbine of the type shown in Fig. l,

Fig. 3 is a schematic illustration of the temperature sensing servo mechanism,

Fig. 4 is a graphic illustration `of engine operating curves,

Fig. 5 is a block diagram of the engine and fuel control combination, and,

Fig. 6 illustrates curves for fuel ow and compressor discharge pressure at various speeds.

Referring to Fig. l, a typical helicopter installation for a turbine type power plant is illustrated. The helicopter may have a main rotor 10 including variable pitch blades 12 whose pitch can be controlled by linkage 14 and in turn moved by main pitch control link 16. The

particular operation of the various parts is more clearly 60 illustrated in copending patent application 388,508, filed October 27, 1953. v

As seen herein, ar turbine type power plant -is generally indicated at 18. The power plant has a first stage turbine 20 which drives the compressor 22. A second-stage tur- 65 bine 24 has no connection to the turbine 20 or the compressor 22 but itdrives the rotura-10 via the shaft 26, gears 28 aridi.: shaft? 301 .Since the secondstage turbine 24 has ist-.VCC

35, 36 and couplings 37 and 38. In order to properly control this power plant rotor combination it is necessary to sense the speed of the free turbine 24 (NF) and the speed of the turbine 26 including the compressor 22 and the speed of the latter is hereinafter referred to as (NG). It is also necessary to sense compressor inlet temperature and compressor discharge pressure. The various ways in which these parameters of power plant operation are utilized in controlling a fuel flow is more clearly described in connection with Fig. 2.

The fuel control of this invention is schematically illustrated in Fig. 2. Generally, the control meters fuelas a function of speed multiplied by a function of compressor discharge pressure. lThe speed ofthe free turbine (NF), in other words the turbine that drives the helicopter rotor, is utilized for steady state control. Maximum limiting however is induced as a force in the system as a function of the speed of the turbine which drives the compressor and a function of the inlet air temperature and compressor discharge pressure. These basic parameters of power plant operation each function to control fuel control in a manner to be described hereinafter.

Referring to Fig. 2, fuel under pressure flows into the main line and through a filter 62. The pressure relief valve is generally indicated at 64 and prevents fuel pressure from exceeding a given value. Fuel from the main line 60 is metered through an orifice 66 in the main throttle valve generally indicated at 68. The metered fuel flows past the orifice 66 and then to the line 70. Apressurey regulator valve 72 is shunted across the throttle valve inlet line 66 and the valve outlet line 70. The pressure regulating valve 72 maintains the pressure drop across the metering orifice 66 at a predetermined value. The pressure regulating valve 72 includes a piston 74, a diaphragm 76 and a spring 78. The bottom of the diaphragm 76 is exposed to fuel pressure on the downstream side of the throttle valve. The top side of the diaphragm is exposed to fuel pressure at the inlet side of the throttle valve. Fuelrunder pressure from the main line 60 flows into an orifice and land 80 into piston 74. The piston 74v has a lapped t with its surrounding wall but the lit is not sufficient to prevent passage of fuel to the upper side of the diaphragm 76. By passing high pressure fuel to the upper side of the diaphragm in this manner a damping effect is provided. Metered fuel in the line then flows past a manual shut-olf valve generally indicated at 34. The shut-off valve 84 is operatively connected to the main power control lever S6 by means of a lost motion member 88. The shutoff valve is intended to positively cut olf fuelywhen the power control lever is moved to a closed position. Meteredfuel flows past the shutoff valve 84 into the line 90 and then into the chamber 92 past an -overspeed shutoff valve 96 and then to the power plant. The operation of the overspeed shutolf valve 96 will be described hereinafter.

In the main throttle valve 68 the fuel under inlet pressure passes through a restriction 100 and into the chamber 102 wherein it acts on the bottom of a piston 104. This high pressure fuel flows into line 108 and thence out .of an orifice 110. The rate with which the fuel is discharged from thejoriiice depends upon the position of the lever or valve member 112. For a given opening of the orifice 110 there willv be a certain pressure in the chamber 102 acting on the piston 104. It will be noted that movement of the piston 104'causes movement of the integral throttle valve stem 114 to thereby varythe Varea of the metering orifice 66. The lever 112, which varies the opening of the orifice 110, pivots Vabout thel point 118` and controls the position of the servo piston 164 and hence the opening ofthe throttle y valve. The lever 112 is, Vin turn, positioned by a corri'- bination of two main forces, one of these forces is that produced in the lever 129 and the other force is produced by the lever 122 through its rollers 124. The lever 120 has a-force imposed upon it by meansof a pair of bellows 126 and128. The bellows 126 is evacuated while the inside of bellows 128is exposed to com pressor discharge pressure (P3) so that these bellows impose a force or signal in lever 120 as a function of compressor. discharge pressure absolute. The lever 122 has a force. imposed thereon for a function of speed, or speed and temperature, in a manner to be described hereinafter.

When compressor discharge pressure increases, the force onlever 120 is increased` in adownward direction thereby causing the` rollers 12.4 to move down and the right handv of lever 112 to move downwardly a slight increment thereby decreasing the dow from theV orifice 110. This increases the pressure in theV chamber 102 beneath the piston 104 thereby moving the piston 1114 in an upward direction against the force of the heavy spring 139. This results in an increase of the opening of the main throttle valve. This upward movement of the throttle valve stem 114 and the servo piston 164 causes an increase in force on the right-hand end of lever 112 through the action of a force feedback spring 132, and restores to their balanced position the members 112 and 121).

Referring again to the link 1221 of the force multiplying mechanism, the rollers 121 permit the link 122 to move to the left or right in response to motion of the vertical link 142. This motion of link 122 varies the mechanical advantage of the links 112 and 120 such that there will be a different throttle valve setting for a given force application on the links 112 and 12). It should be addedV that when mentioning that any of the parts have motion this motion is extremely small and the system is primarily a force balance system. Thus if there is any motion imparted to the member 122 the moment arms through which the other forces are acting will change to cause a force unbalance. The throttle valve 114 will then assume a new position to rebalance the forces. Thus motion of 122 toward the right increases the moment arm of the force acting through 120 on rollers 124 and in turn the moment arm of the right side of 112 which acts on spring 132. This unbalance will be reflected in an increase in throttle valve opening and a rebalancing of the system.

The link 112 is moved to the left or right via the link 142 in response to one or more of several parameters of power plant operation depending on whether the control is in steady state operation or under maximum limiting operation such as during acceleration, overspeed or underspeed. The link 122 is also controlled by the vertical link 144 which actuates a pivoted bell crank-type member 146 through link 142. The vertical link 144 is operated by a member 1558 which is pivoted at 150 intermediate the ends thereof and has its left end terminated in a bifurcated portion 152. The portion 152 is engaged by a roller 154 which is backed up by a spring 156 and is located at the right-hand end of a bar S. The bar 158 is pivoted about a cam 160 which cam is rotated by the power control lever 86. The left-hand end of the bar 158 is connected by a link 162 to a servo motor generally indicated at 164. The servo piston 182 provides a motion proportional to speed of the free turbine (NF) such that combined with the desired speed setting of the cam 161), the right-hand en'd of the bar 158, including the roller 154, is moved, rotating link 148 about pivot 150 and thereby translating Vertical rod 144 which then would be positioned proportional to speed error. This motion or speed error in turn is transmitted through the bell crank 146 and to the rod or link 122 to the throttle mechanism so as to adjust the throttle valve in a manner to ybe described below. This speed error` signal is the main governing signal of the fuel control. l

This governing is accomplished by the governor generally indicated at 163 in the lower left-hand corner of Fig. 2. The yball governor 168 is driven by the free turbine or the turbine which drives the rotor of the helicopter, or the propeller in the case of a turboprop type installation. The ilyball force of the governor 168 is opposed by a spring 170 such that the servo valve 172 moves up or down in responseto a force unbalance allowing either high pressure liuidv tov ow from the line 174 to the line 176, then to the line 178, to the chamber 1811, to the bottom of the servo piston 182; or low pressure uid to flow from line 178 through the pilot valve to line 184. It should be noted that the chamber 132A of the servo pistonh182 is connectedcontinuously to high pressure uid via the line 1186. Y Y' v' Movement ofthe servo` piston 18a?,v readjusts the force exerted by the spring 176 through the operation of lever 1&8 to rehalance the force exerted by the governor iiyballs and stop the motionv of the servo valve A17.2and1 the servo piston 182. From the foregoing, it is apparent that the position of the servo piston 1&2 will at all times be proportional to the free turbine. speed (NF). it is further apparent then that the left-hand end of the longitudinal bar 15S is subjected to a position proportional to free turbine speed while, intermediate the ends of the bar 158, the cam 16u is positioned in proportion to desired speed whereby the right-handend of the bar 158 provides a signal proportional tothe speed error at any particular instant. The right-hand end of bar 158 including its roller 154 sends the speed error signal through the bifurcated end 152 of link 148 tothe vertical link 144 etc. to position the throttle valvel accordingly. To follow a typical operation of the speed signal, consider the condition where the load on the helicopter rotors is suddenly reduced for some reason which causes the rotor and the free. turbine to overspeed. In this event, the ilyballs of the governor 168, will move outwardly forcing the servo valve 172 in an upwardy direction thereby allowing high pressure fluid to act on the body of the servo piston 132 and moving the servo piston upwardly. Through the action of the lever 133., the spring 170. will be compressed thereby rebalancing the fiyball force and' stopping the motion of the pilot valve 172 and the servoV piston 182. However, the new position of the servo piston 182 will be transmitted through'the link 162 to the bar 158 to vary the position of the roller 154 on the bifurcated end 152 of link 14.8. This motion rotates 'lever 143 counterclockwise on pivot 150 moving the, rollers 124 toward decrease fuel flow per unit compressor discharge pressure through action of links 144, 146, 142 and 122.

The steady state governing assembly includes an overspeed valve 96 which is connected to the Vbottom. of the servo piston 182. For a given overspeed condition and upward movement of the servo piston 182,` the valve 96 will assume a closed position so as to prevent flow of fuel to the power plant.

This shutoff valve 96 is necessary in a helicopter installation inasmuch as the relative inertia of the power plant and rotor are such that dangerous overspee'ds might readily cause disintegration of the power plant if the normal fueling control elements had to be relied upon to suddenly decrease fuel flow under certain conditions; thus, the position of the servo piston 182 is the best indicator of an overspeed condition and acts substantially instantly to regulate or shut off the flow of fuel.

As previously mentioned, the rollers 124 and the link 122 are also subject to motion from the vertical llink 142 (which is pivoted intermediate its ends at192) during acceleration limiting. The lower end of link 142 is engaged by a rod 194 which has its left end in engagement with a cam 196. The cam 196 is both reciprocable along its vertical axis and also rotatable about that axis. To the left of the cam 196 a lgovernor is ygenerally indicated at 19S. The .governor 198 senses the speed of the .compressor and, of course, the turbine which drives 'the compressor. The governor 198 exerts a force on a lever 200 by means of the vertical member 202. The lever 200 is pivoted to the member 202 and the lever 200 has its right-hand end in engagement with a spring 204 while the left-hand end of the lever 200 varies the opening of an orifice 206. Hence, any variation in compressor speed changes the area of the orifice or valve 206 thereby changing the pressure in the chamber 208 at the lower most portion of the three-dimensional cam 196. The pressure inthe chamber 208 controls the position of a piston 210 which, in turn, moves the cam 196 vertically. The chamber 212 on the upper side of the piston 210 is continuously fed high pressure fuel via a line 214. This high pressure fuel passes through a restriction 216 into piston 210 and thence to chamber 208. The pressure in the chamber 208 is varied in accordance with the opening of the orifice 206 which opening, in turn, `is controlled via the governor 198. As in the previously described servo mechanism, any change in position of the servo piston 210 and its attached threedimensional cam 196 will vary the compression of the spring 204 to rebalance the system, that is7 to restore the opening of the orifice 206 whereby the system is in balance. Thus, the three-dimensional cam 196 has a different axial position for every compressor speed.

The three-dimensional cam 196 is rotated about its vertical axis through a gear 220. The mechanism for rotating the cam is schematically illustrated in Fig. 3. Thus, compressor inlet air is circulated through the lines 224 and 226 so that the bellows 228 expands or contracts in relation to the temperature of the inlet air. The bellows 228 is linked to a vertical rod 230 which at its lower end is connected to a servo valve 232. The servo valve 232 varies the opening of the orifice 234 which is receiving high pressure fluid from a line 236. This high pressure fuel flows to the right-hand side 238 of servo piston 240, but, the valve 232 and the oritice 234 control the pressure to the left-hand side 242 of the piston 240. Hence, any variation in position of the valve 232 varies the position of the servo piston 240 and its integral rack 244 thereby rotating the gear segment 246 which is equivalent to the gear 220 on the three-dimensional cam of Fig. 2.

The three-dimensional cam 196 also includes at its upper end cam surfaces 250 and 252. These surfaces are intended to engage the left-hand end of lever 254 andthe arm 256 respectively. The arm 256 is integral with a vertically movable rod 258. It is thus seen that during limiting the cam surface 196 can move the horizontal rod 194 and the cam surfaces 250 and 252 can aiect the rods 254 and 256 respectively.

Assuming, for example, that the compressor speed has increased to a value where it is to be limited from further increase for optimum engine performance and safety, the cam surface 250 will engage the left end of the topping lever 254 and will rotate the lever 254 clockwise about its pivot 260 thereby also causing the rod 148 to move counterclockwise about its pivot 150. This motion is transmitted to the vertical link 144 and bell crank y146 to the horizontal link 122 to move it in a decreased fuel ilow per compression discharge pressure direction.

Movement of the main cam 196 in a downward direction in response to a decrease in compressor speed ,to-V

ward a dangerously low r.p.m. will cause the cam surface 252 to engage the arm 256 and cause downward motion thereof along with the rod 258. This eventually causes the rod 286 to rotate counterclockwise about its pivot 266 thereby forcing the left-hand end of the bar 148 upwardly. This motion is transmitted to the vertical link 144 through a member 146 and then to the horizontal link 122 which is moved in a fuel per unit compressordischarge pressure increase position. .Thus the control provides a topping and bottoming function to prevent excessive speed or too low a-speed.

Maximum and minimum fuel flow limiting for a transient operation of the engine is described immediately following. During an acceleration of the engine it isl desirable to have the largest fuel liow per unit compressor discharge pressure possible in order to provide fast response to the pilots motion of the power lever. In order to preventl compressor surge and overtemperature, however, it is necessary to limit the maximum fuel flow for given conditions. Herein, the limiting is accomplished as a schedule of fuel flow in proportion to a function of compressor speed, compressor inlet temperature and compressor discharge pressure. Thus, as seen in the lower right-hand corner of Fig. 2, the horizontal link 194 assumes a position as a function of compressor speed and compressor inlet temperature through the main cam 196. The link 194 iny turn transmits a position to the vertical link 142 which imposes a limit on the horizontal link 122 to prevent its .further movement toward an increase fuel iiow per unit compressor discharge pressure. Therefore, if the power control lever S6 calls for an increase speed through its cam 160, the right-hand end of the rod 158 tends to move upwardly thereby rotating rod 148 clockwise about its pivot 150 thereby permitting the vertical link 144 to move downwardly and the main horizontal link 122 to move toward an increase fuel flow per compressor discharge pressure. This increase fuel yflow per unit compressor discharge pressure signal, however, will be limited by the particular position of the limiting links .i194 and 142, depending upon the particular compressor speed and inlet temperature existing at that instant. l

Minimum fuel flow per compressor discharge pressure is established by a stop 28@ which engages the left-hand end of link 148. A negative feedback link identiied in Fig. 2 is provided for reasons described immediately hereafter. The `operation of the negative feedback link is best described by referring to the curves shown in Fig. 4. The standard plot of the ratio of fuel flow to compressor discharge pressure (WF/P3) vs. speed (in this lcase the speed of the engine compressor) is shown. Line A represents the maximum allowable fuel flow per compressor discharge pressure for engine acceleration as established by the characteristics `ofthe engine for prevention of pressure surge and overtemperature conditions. This is the fuel flow limit that is accomplished in the control by the main cam 196 of Fig. 2. Line B represents fuel iiow limiting as accomplished by means of the minimum yflow stop 280. Line C is the steady state characteristic ofthe engine. Lines D and E are established byythe bottoming and topping functions of the control respectively. Lines F, G, H, l and J represent lines as established by the free turbine governor 168 of Fig. 2.

Assuming that the pilot calls for rotor speed by means of rotating speed setting cam in a suitable manner the engine will operate at point K (Le. .100% engine speed) if the expected design load is present at the rotor of the helicopter. if the load on 'thehelicopte rotor and consequently the load on the power or free turbine should decrease, the rotor and turbine combination would tend to overspeed and the governing mechanism would call 4for less fuel liow in order to correct for said overspeed. if, for example, a 1% overspeed should occur line G on Fig. 4 would represent the fuel flow vs. compressor discharge pressure ratio which the governing portion of the control would call for, if a 2% overspeed v occurred line H would represent the fuel flow ratio called for by the control, and so on. In the case of a 1% overspeed-resulting from a decrease in load on the helicopter rotor and power turbine combination, speedkof the engine would bedecreased such that operation would now occur at point L on the engine steady state line. Similarly,.for an overspeed of 2% the engine would operate at point M on the steady state line., Now if an overspeed of `slightly more than 2%, say 2.2% should occur, a certain ifuel flow ratio as evidenced by line Q would be called for and the engine would be expected to operate at the point where said line intersects the engine steady state curve. lt is this region of engine operation represented by the steady state curve between line P and the bottoming line D where an unstable condition could exist. Thus a means yfor providing a greater degree of stability becomes desirable. Lines R, S, T, and U represent such a provision and are accomplished in the control by means of the negative feedback link and its attending mechanism.

Still referring to Fig. 4, let it be assumed that the gow erning portion of the control is calling for a decrease in fuel flow per unit compressor discharge pressure as a result of an overspeed of the power turbine and that the speed of the engine is decreasing toward a value represented at line P. The bottoming cam surface of Pig. 2 will contact the arm 256 and via the rod 25S will move the left-hand end of the negative feedback in a downward direction thereby allowing link 284, which engages the feedback link, to move in a downward direction. This moves the pivot 150 downwardly thereby permitting the right side of rod T43 to move in a downward direction thus calling for an increase in fuel flow per unit compressor discharge pressure at a rate which is increasing with a decrease in engine speed. This provides the characteristic as indicated by lines R, S, T, and U. If the overspeed is of the magnitude such as indicated by lines I and R, the negative feedback line will intersect the bottoming governor line which in turn will limit the decrease in engine speed. If the speed of the engine decreases to the value represented by point L in Fig. 4 the shoulder (Fig. 2) 285 of vertical rod 258 will contact the bottoming lever 286. which in turn, causes the bar 143 to move downward at its right-hand end and pivot about 159 to sharply increase fuel flow per unit compressor discharge pressure.

The necessity of the negative feedback provision is explained by the consideration of the response times of the various elements in the system in connection with a consideration of engine characteristics in respect to fuel consumption and the variation of compressor discharge pressure over a speed range.

Looking at Fig. 5 and keeping in mind the nature of the connection between each box of the block diagram it will be evident that two of said connections are of a relatively slow acting type. Since the only connection between the engine and the free turbine is one o-f an aerodynamic nature, response of the free turbine to change in engine speed will be slow in comparison with the response of other elements. Likewise, since the signal sent back to the speed governor which is responsive to the speed of the free turbine is dependent upon the response time of said free turbine, it will have a relatively slow response time. The effect of feeding the compressor discharge pressure back to the throttle valve and multiplier is to effectively increase the gain of the system since the feedback is of a positive nature. if, for example, the speed governor calls for an increase in fuel ow per unit compressor discharge pressure as a result of an underspeed existing in the free turbine, as the engine speed increases the compressor discharge pressure signal fed back to the multiplier will likewise increase since an increase in engine speed causes an increase in compressor discharge pressure. The effect or". the compressor discharge pressure feedback therefore is to further increase the ow of fuel to the engine which effect occurs because of the delay in the signal indicative of a changed free turbine speed reaching the speed governor to call lfor a decrease in fuel liow. lt will therefore be seen that if for any region o-f the engines operation the compressor discharge signal which is being fed back to the engine multiplier and throttle valve is of an unduly large magnitude unstable system operation will result from the extremely high gain caused. This may occur in the region enclosed by the lines P and D of Fig. 4 for the following reasons:

Fig. 6 illustrates the engine characteristics as regards fuel consumption and compressor discharge pressure over a similar speed range. From an examination of the two curves it will be seen that at a relatively high speed the change in fuel flow for` a lgiven change in speed is of a magnitude simil-ar to the magnitude of the change in compressor discharge for the same change in speed. However, at lower 'speeds (enclosed by lines P and D of Fig. 3) the change in compressor discharge pressure for a given change in speed is considerably larger than the change in fuel flow for the same change in speed. Thus, at these lowspeeds the compressor discharge pressure signal (referring again to Fig. 5) fed back to the throttle valve and multiplier has a larger effect for a given speed error than its effect for the same speed error in a higher speed region.

The negative feedback provision previously described, acts to decrease the magnitude of the speed error signal entering the throttle valve and multiplier unit, thereby decreasing the gain of the system and tending to balance the effect of the compressor discharge pressure feedback in the critical speed region. Thus a more stable operation is provided by its incorporation. Other methods for accomplishing the same purpose would be to incorporate a lag in the compressor discharge pressure feedback or to improve the response time of the free turbine and thereby provide a faster subtracting signal for the speed governor. lt should be noted that the necessity of incorporat ing the feedback or other corrective measures arises only because in the particular application at hand it is necessary to operate the engine in the critical speed region and if it were possible to restrict operation of the engine to the right of line P in Fig. 4 the provision of feedback or other means similar would probably not be necessary.

Referring again to Fig. 2 it will be noted that' the main three-dimensional cam 196 is engaged on its left side by a follower 3d() which in turn actuates a bell crank 302 and a rod 304. The rod 304 is intended to provide another signal which is a function of compressor speed and compressor inlet temperature. This signal may be sent through a servo device and may, for example, control some member or portion of the power plant. A typical use for such a signal is to vary the angle of the stator blades in the compressor for different operating conditions.

As a result of this invention, it is apparent that a fuel control has been provided having particular novel features.

Although only one embodiment of this invention has been illustrated and described herein it will be apparent that various changes and modifications may be made in the construction and arrangement of the various parts without departing from the scope of this novel concept.

What is desired by Letters Patent is:

l. In a fuel control for a turbo power plant having a compressor, a combustion section for producing gases, a first turbine driven by gases produced by said combustion section for driving the compressor, a second turbine receiving gases produced by said combustion section, a source of fuel under pressure, means for regulating the ow of fuel from said source to said combustion section, means responsive to a variable of power plant operation which effects power output for producing a rst signal, means responsive to the speed of said second turbine for producing a second signal, means for combining said signals to control said regulating means, means responsive to the speed of said rst turbine, means responsive to the temperature at a predetermined point in the power plant, a cam movable in two directions in response to said last mentioned speed responsive means and said temperature responsive means, said cam being operable to limit the maximum fuel How permitted by said regulating means, and a second cam for controlling said regulating means to limit the maximum and minimum speed of said firstturbine.

2. In a -fuel control according to claim 1 comprising a mechanism whereby said second cam is movable in response to said last mentioned speed responsive means and said temperature responsive means.

3. In a fuel control for a turbo power plant having a compressor, a combustion section for producing gases, a rst turbine receiving gases produced by said combustion section for driving the compressor, a second turbine produced by said combustion section, a source of fuel under pressure, means for injecting fuel into said combustion section including fuel regulating mechanism between said source and said combustion section, a first means responsive to the pressure at a predetermined point in the compressor for producing a first signal, means responsive to a speed of one of said turbines for producing a speed signal, means for producing a signal commensurate with desired speed, means for comparing said speed and desired speed signals for producing a speed error signal, means for multiplying said first signal and said speed error signal and controlling said fuel regulating mechanism, means responsive to the speed of the other of said turbines and the temperature of the air at the inlet to the power plant for modifying said first signal to limit the fuel flow regulation of said regulating mechanism, and means for limiting the maximum and minimum speed of one of said turbines as a function of compressor inlet temperature.

4. In a fuel control according to claim 3 wherein said last mentioned means includes a mechanism for limiting the fuel flow and hence the speed of one of said turbines as a function of the speed of the other of said turbines including operative connections to said fuel flow regulating mechanism.

5. In a helicopter having a rotor, a turbine type power plant for driving said rotor including a compressor and a combustion section for producing gases, a first turbine receiving gases produced by said combustion section and `driving said compressor, a second turbine receiving gases produced by said combustion chamber and driving said rotor, a source of fuel under pressure, means for regulating the flow of fuel lfrom said source to said combustion chamber, means responsive to the speed of said second turbine for producing a first `signal including speed selecting mechanism operatively connected thereto, means responsive to the speed of said first turbine for modifying said first signal including a three-dimensional cam and a servo device for moving said cam in `one direction, power plant inlet temperature responsive means for also modifying said first signal including a servo device Ifor moving said cam in another direction, means responsive to compressor discharge pressure for creating a second signal, vmeans for multiplying said signals for controlling said regulating means.

6. In a helicopter having a rotor, a turbine type power plant for driving said rotor including a compressor, a combustion section for producing gases, a first turbine receiving gases produced by said combustion section for driving said compressor, a second turbine receiving gases produced by said combustion section and driving said rotor, a source of fuel under pressure, means for regulating the flow of fuel from said source to said combustion chamber, means responsive to the speed of said second turbine for producing a first signal including speed selecting mechanism operatively connected thereto, means responsive to the speed of said first turbine for modifying said first signal including a three-dimensional cam and a servo device for moving said cam in one direction, temperature responsive means for also modifying said first signal including a servo device for moving said cam in another direction, means responsive to movement of said cam a predetermined distance in said one direction for further modifying said first signal. to prevent the speed of said first turbine from going beyond a predetermined value, means responsive to compressor discharge pressure for creating a second signal, and means for multiplying said signals for controlling said regulating means.

7. In a fuel control for a turbo power plant having a compressor, a combustion section, a first turbine receiving lgases produced by said combustion section for driving the compressor, a second turbine also receiving gases produced by said combustion section, a source of fuel under pressure, means for injecting fuel into said combustion section including a fuel regulating mechanism between said source and said combustion section, a first means responsive to the pressure at a predetermined point in the compressor for producing a first signal, second means responsive to speed of one of said turbines for producing a second signal, means for combining said signals and controlling said fuel regulating mechanism, means responsive to the speed of the other of said turbines and the temperature of the air at the inlet to the power plant for modifying one of said signals to limit the fuel flow regulation of said regulating mechanism, and means for limiting the maximum and minimum speed of one of said turbines as a function of compressor inlet temperature.

8. In a fuel control according 'to claim 7 wherein said last-mentioned means limits the fuel flow and hence the speed of said last mentioned turbine as a function of the speed of said first turbine including operative connections to said fuel flow regulating mechanism.

9. In a fuel control for a turbo power plant having a compressor, a combustion section for producing gases, a turbine driven by the gases from said combustion section for driving the compressor, a second turbine driven by gases exhausted from said first turbine and driving a variable load, a source of fuel under pressure, rneans for injecting fuel into said combustion section including a throttle valve between said source and said combustion section, a first means movable in response to compressor discharge pressure at a predetermined point in the compressor for producing a first signal, a second means movable in response to the speed of rotation of one of said turbines for producing a second signal, means operatively connected to said throttle valve and said first and second means for multiplying the signals of said first and second means and varying the opening of said throttle valve in proportion to the product of said signals including a servo operated device for moving said throttle valve, and means responsive to the speed of the other of said turbines, said last-mentioned means being operatively connected to said throttle valve for limiting the throttle valve opening.

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